Ask Mr. Science
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 artist's rendering of black hole
Black holes

I did the standard demonstration that shows how large masses distort space and attract other masses. I still owned a screen-printing screen back from the days when I was an artist. It is fine-mesh nylon stretched over a 2x3' frame. If you lay the frame on the table, and carefully level it with a level and some wedges, you have a model of flat space. The nylon is stretched taut enough so that a small weight will not noticeably depress it.
A marble or 1/2' steel ball will roll in a straight line across the cloth without deviation, just as a spaceship, with the engines off and not too close to a star or planet, will float through outer space in a straight line. Of course our model is in 2 dimensions only. Now we hang a massive body like a star in our toy space. The most appropriate object I had on hand was a potato-sized smooth river rock. When placed on the nylon, it depresses the surface 1/2' or so right under the rock, flattening out again towards the edge of the frame. Now, when the marble is placed on the screen, it will be attracted to the rock. Also, if you roll the marble on a trajectory close by the rock, its path will be noticeably deflected. With some practice, you can even get your marble to go into orbit around the river rock.
This model for space distorted by massive objects is such a standard, that you see it pictured in many sci-fi movies. It usually shows up as a square grid of lines that the hero's spaceship is zooming along, and black holes show up in these animations as funnel-shaped holes in this grid, down which our hero is sucked, spaceship and all. The more massive the rock is, the stronger the depression, and the stronger the attraction, and the faster the marble rolls towards the center.
... more on this...

3 March 98, 24 Nov 98


 

aurora
Electricity and magnetism

I spent two weekly hours on the subject. First, I wanted to show that most of the electricity around us comes from 2 different sources: batteries and generators.
For the batteries, I did the standard stuff: any acid plus any two metals make a battery. I had an orange, a lemon, and some scraps of cloth with vinegar. For different metals, I had thick copper wire, big galvanized nails (for a zinc surface), steel nails, aluminum foil, pennies. Then, with some telephone wire and my old analog voltmeter, we made several batteries: the standard one where you stick nails and wire of different metals into the lemon or orange. We noted the different voltages, and hooked them up in series to get a little higher. Unfortunately, one or two lemons is not enough to light up a flashlight bulb. We made a 'Volta pile' (reference to the original pile here) by stacking aluminum foil, vinegar-soaked cloth and pennies.
For the generator part, I had made a simple generator:

simple generator
  • In a board, I screwed four 3" drywall screws in a 1.25x2" rectangular pattern. Around this I wound a bunch of telephone wire.
  • I had a pretty strong fridge magnet, about 2x1x3/8". I fixed an axle to it by bending a piece of coathanger wire, and tying it in place with string. The wire stuck out about 2" on either side.
  • Two pieces of cardboard, folded, glued to the wood base, and with slots in the top to hole the magnet, formed pretty good bearings.
  • Slip the magnet into place, wrap masking tape around the axle right next to the cardboard, so that the axle cannot slip sideways. The masking tape also make a fatter place for your fingers to grab onto when you twirl the magnet.
  • Of course, the dimensions given here all depended on the particular magnet you happen to have around. The goal is to have the wire as close to the twirling magnet as possible, while at the same time have enough clearance so that the magnet can twirl freely.


Twirling the magnet produces enough voltage to register easily on the voltmeter (as AC, not DC, but I glossed over that detail), but again not enough for a flashlight bulb. [Would an LED work?]

een voorvork, voorwiel, dynamo en achterlichtje
Here's a closeup
The next week I realized that I did have a generator that would light up a lamp. Being a Dutchman, I had in my box of bike parts an ordinary 'dynamo', or bike generator. This thing mounts on the front fork of your bike, and when engaged, presses against the tire. Out of the same box came a front bike fork, and to complete the demonstration, I borrowed a wheel from my garden cart, which I had built using 2 front bicycle wheels. I could mount the whole assembly upside down with some scraps of wood, and light up a bike light brightly with even a gentle turn of the wheel.
In the real world, what turns generators? We talked about dams, and generators powered by oil, gas, coal and nuclear power. The children were clever enough to add solar and wind power to the list. They were well aware of the fact that fossil fuels cause plenty of environmental problems, but not that these fuels were going to run out in their lifetimes, and that they had a big problem to solve when they grew up to be engineers.

31 March, 7 April 98, 2 and 9 Feb 2000


 

aurora
What is the Aurora?

Time to talk about the sun and all that. Most of what I talked about is on the first two web sites on the list below: How the sun is not as quiet and peaceful as it may seem, that great blasts of material are blowing away from the surface all the time, and that this solar wind roars by the earth. How the Earth is like a big bar magnet that channels all that stuff to the North and South poles. That the space shuttle typically flies lower than the Aurora. One of the harder concepts (for these 3rd-graders) was that the atmosphere does not just stop at a well-defined altitude, but that there is (very) thin gas up where the shuttle flies, and higher still.
Check out the top 2 links below!

To show that the sun is not without blemishes, I also had brought in my home-built sunspot viewer, and we took it outside and observed a fair number (5 or so) sunspots that day. More on the sunspotter below.

14 April 98, 17 November 99








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